The present invention relates to an isomerization process and the prevention of the deactivation of an isomerization catalyst used in such process.
Isomerization of normal alkanes, i.e., normal paraffins, is widely used in refinery processes for the upgrading of lower-valued hydrocarbons to hydrocarbons of higher value. In recent years there has been an increased interest in the isomerization of normal alkanes, having about 4 carbon atoms to about 10 carbon atoms, to isoalkanes, particularly the isomerization of normal butane to isobutane, and also, normal hexane to 2,2-dimethylbutane. Due to recent federal mandates concerning the vapor pressure of gasoline, it is desirable that high vapor pressure components, such as normal butane and normal hexane, are removed from the gasoline pool. However, upon the removal of such high vapor pressure components there must be some other use for such components. Thus, for example, butane isomerization is beneficial because isomerization of n-butane produces isobutane which can be used as a feedstock for various other refinery processes, such as alkylation and etherification, that produce high octane gasoline components.
The use of supported platinum catalysts (such as platinum on alumina) for isomerizing hydrocarbons, in particular normal alkanes to isoalkanes (such as n-butane to isobutane), is well known. A problem that is encountered in the isomerization of hydrocarbons is the rapid deactivation of the isomerization catalyst. There are believed to be a number of causes of catalyst deactivation. One such cause of catalyst deactivation is the formation and accumulation of high molecular weight hydrocarbons, such as C.sub.5 to C.sub.8 hydrocarbons, carbon, and/or coke, within the pores of the isomerization catalyst, particularly at the reaction sites, also referred to as acid sites, within the isomerization catalyst as well as on the isomerization catalyst surface. The formation and accumulation of such high molecular weight hydrocarbons causes a high rate of catalyst deactivation, a short run life of the catalyst, and an unsteady yield of hydrocarbon products.
In addition, impurities present in the feed stream contribute to a rapid decrease in catalyst activity. Pretreatment of the feed stream prior to isomerization to remove a major portion of these impurities is one option to help alleviate catalyst deactivation, but this route is expensive because additional equipment and operating costs are required. Also, the levels of these impurities in the feed stream may fluctuate, and pretreatment of the feed stream may not always be adequate.
Another option to alleviate the deactivation of isomerization catalysts by impurities is to operate the isomerization processes at relatively high hydrogen to hydrocarbon ratios and at relatively high temperatures. However, this route is also expensive and generally produces less amounts of the desirable isomer product(s) and more amounts of the undesirable by-products, mainly light gases which are formed by hydro-cracking of feed hydrocarbons.